[vm] Fix crash when --heap_snapshot_on_oom triggers on the background compiler.

Remove use of Thread::Current()->isolate().

TEST=ci
Change-Id: I991d4339cc81e11b42a691f3884eb0051c829d61
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446985
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: rmacnak-google

runtime/lib/developer.cc

@@ -212,6 +212,7 @@ DEFINE_NATIVE_ENTRY(Developer_NativeRuntime_writeHeapSnapshotToFile, 0, 1) {
       String::CheckedHandle(zone, arguments->NativeArgAt(0));
   bool successful = false;
   {
+    NoActiveIsolateScope no_active_isolate_scope(thread);
     FileHeapSnapshotWriter file_writer(thread, filename.ToCString(),
                                        &successful);
     HeapSnapshotWriter writer(thread, &file_writer);

runtime/vm/heap/heap.cc

@@ -196,6 +196,7 @@ uword Heap::AllocateOld(Thread* thread, intptr_t size, bool is_exec) {
   if (FLAG_heap_snapshot_on_oom != nullptr) {
     bool successful = false;
     {
+      NoActiveIsolateScope no_active_isolate_scope(thread);
       FileHeapSnapshotWriter file_writer(thread, FLAG_heap_snapshot_on_oom,
                                          &successful);
       HeapSnapshotWriter writer(thread, &file_writer);

runtime/vm/object_graph.cc

@@ -1501,7 +1501,7 @@ void HeapSnapshotWriter::Write() {
 
   WriteBytes("dartheap", 8);  // Magic value.
   WriteUnsigned(0);           // Flags.
-  WriteUtf8(isolate()->name());
+  WriteUtf8(isolate_group()->source()->name);
   Heap* H = thread()->heap();
 
   {
@@ -1525,7 +1525,8 @@ void HeapSnapshotWriter::Write() {
     Library& lib = Library::Handle();
     String& str = String::Handle();
 
-    intptr_t field_table_size = isolate()->field_table()->NumFieldIds();
+    intptr_t field_table_size =
+        isolate_group()->initial_field_table()->NumFieldIds();
     const char** field_table_names =
         thread()->zone()->Alloc<const char*>(field_table_size);
     for (intptr_t i = 0; i < field_table_size; i++) {
@@ -1707,7 +1708,7 @@ void HeapSnapshotWriter::Write() {
     iteration.IterateObjects(&visitor);
 
     // External properties.
-    isolate()->group()->VisitWeakPersistentHandles(&visitor);
+    isolate_group()->VisitWeakPersistentHandles(&visitor);
 
     // Smis.
     for (SmiPtr smi : smis_) {
@@ -1796,7 +1797,7 @@ void HeapSnapshotWriter::Write() {
 
     // External properties.
     WriteUnsigned(external_property_count_);
-    isolate()->group()->VisitWeakPersistentHandles(&visitor);
+    isolate_group()->VisitWeakPersistentHandles(&visitor);
   }
 
   {